1. Field of the Invention
This invention concerns a method and an apparatus for liquefying natural gas. In another aspect, the invention concerns an improved multi-stage expansion cycle for reducing the pressure of a cooled and pressurized liquefied natural gas (LNG) stream to near atmospheric pressure.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Typically, LNG plants that employ an open methane cycle for the final refrigeration cycle utilize three expansion (i.e., flash) stages, with each expansion stage including flashing of the LNG-bearing stream in an expander followed by separation of the flash gas stream and LNG-bearing stream in a gas-liquid phase separator. In a conventional open methane cycle, the final flash stage includes reducing the pressure of the LNG-bearing stream to about atmospheric pressure in a final-stage expander and then separating the low pressure flash gas stream from the low pressure LNG-bearing stream in a final-stage gas-liquid separator. From the final-stage separator, a cryogenic pump is used to pump the low pressure LNG-bearing stream to the LNG storage tank(s).
As in all processing plants, it is desirable for LNG plants to minimize capital expense and operating expense by reducing the amount of equipment and controls necessary to operate the plant. Thus, it would be a significant contribution to the art and to the economy if there existed an open methane cycle that eliminated at least some of the equipment and/or controls associated with the multi-stage expansion cycle.
It is an object of the present invention to provide a novel natural gas liquefaction system that employs an open methane cycle and requires a reduced amount of equipment.
Another object of the invention is to provide an open methane cycle that does not require cryogenic pumps to transport the LNG-bearing stream from the final-stage gas-liquid separation vessel to the LNG storage tank.
A further object of the invention is to provide an open methane cycle that utilizes only two separation vessels rather than the conventional three separation vessels.
It should be understood that the above objects are exemplary and need not all be accomplished by the invention claimed herein. Other objects and advantages of the invention will be apparent from the written description and drawings.
Accordingly, in one embodiment of the present invention there is provided a process for liquefying natural gas comprising the steps of: (a) flashing a pressurized liquefied natural gas stream in a first expander to provide a first flash gas and a first liquid stream; (b) flashing at least a portion of the first liquefied stream in a second expander to provide a second flash gas and a second liquid stream; and (c) flashing at least a portion of the second liquid stream at or immediately upstream of a liquefied natural gas storage tank, thereby providing a third flash gas and a final liquefied natural gas product.
In another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) flashing a pressurized liquefied natural gas stream in a first expander to provide a first flash gas and a first liquid stream; (b) flashing at least a portion of the first liquid stream in a second expander to provide a second flash gas and a second liquid stream; (c) subcooling at least a portion of the second liquid stream in a heat exchanger, thereby providing a subcooled liquefied natural gas stream; and (d) conducting at least a portion of the subcooled liquefied natural gas stream to a liquefied natural gas storage tank.
In a further embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) flashing a first liquefied natural gas stream in a first expander to provide a first flash gas and a first liquid stream; (b) conducting a product portion of the first liquid stream to a liquefied natural gas storage tank; (c) conducting a refrigerant portion of the first liquid stream to a heat exchanger; (d) conducting natural gas vapors from the liquefied natural gas storage tank to the heat exchanger; and (e) combining the natural gas vapors and the refrigerant portion in the heat exchanger.
In still another embodiment of the present invention, there is provided an apparatus for liquefying natural gas. The apparatus comprises a first liquid expander, a first gas-liquid separator, a second liquid expander, a second gas-liquid separator, an indirect heat exchanger, a splitter, and a liquefied natural gas storage tank. The first gas-liquid separator is fluidly coupled to an outlet of the first expander. The second liquid expander is fluidly coupled to a liquid outlet of the first gas-liquid separator. The second gas-liquid separator is fluidly coupled to an outlet of the second expander. The indirect heat exchanger defines a first fluid flow path and a second fluid flow path that are isolated from one another. The first flow path inlet is fluidly coupled to the second liquid outlet. The splitter is fluidly coupled to an outlet of the first flow path. The liquefied natural gas storage tank has an inlet that is fluidly coupled to a product outlet of the splitter.